Lycophytes and Ferns

1. embryos
2. roots, stems, leaves
3. xylem and phloem (vascular tissue)
4. trend toward less and less dependence on external water for vegetative growth and sexual reproduction (contrast mosses with angiosperms - thick cuticle, pollen vs. free-swimming sperm, etc.)

• Comparison of the habits of Lycopodium dendroideum (right) and Diphasiastrum digitatum (left).
• Diphasiastrum, showing small leaves (microphylls) that are alternate ternate, opposite or whorled, simple, one-veined.
• The sporangia are sometimes aggregated into a cone-like strobilus.  In other members of this family, such as Huperzia, the sporangia are not in strobili but occur near the base of the sporophyll.
• Sporangia borne in axils of fertile leaves called sporophylls [figure])
• A section through the strobilus shows that the plants homosporous (i.e. produce one size of spore).

• Habit of Selaginella which has dichotomously branched stems, leaves alternate, opposite or whorled, simple, one-veined, sometimes dimorphic (two sizes), with scale-like ligule (early deciduous). 
• Roots borne on wiry rhizophores arising from forks in stems.
• Sporangia borne in axils of fertile leaves (sporophylls). Plants heterosporous [section through mega- and microsporangia]. Microsporangia and megasporangia can be borne in the same or different strobili.
• Microspores small, numerous, megaspores large, 4 per megasporangium. 
• The gametophyte develops inside the megaspore! This image shows a young sporophyte developing from megaspore.  This is not a seed!

1. Sporophyte generation.

a. Fern leaf (called a frond) unfolds via circinate vernation ("fiddleheads") from a rhizome. The leaflets of the frond are called pinnae, subdivisions of pinnae are pinnules. These are arranged on a central axis called the rachis.

b. On the back (abaxial) side of the frond are clusters of sporangia called sori. The sori may or may not be covered by an indusium, a flap of tissue that protects the sorus. Sometimes a portion of the leaf forms a false indusium. The sorus is comprised of a few to many sporangia. This longitudinal section of sorus is Polystichum that has an indusium.  This section of Polypodium lacks an indusium.  Indusium Types.

2. Gametophyte generation.

a. Meiosis takes place in the young sporangium in the spore mother cell (2N). The four haploid cells resulting from meiosis divide mitotically to formfirst 8, 16, 32, and then (usually) 64 spores. This marks the beginning of the haploid generation.

b. The sporangium has special thickened cells along one side which react to changes in humidity. The spores are dispersed by the rapid movement of the dehsicing sporangium.

c. The spores land on a suitable (moist) substrate and germinate. The threadlike protonema has chloroplasts and continues to grow via mitotic divisions an apical cel. Eventually, a large, heart-shaped prothallus is formed.  It may be male or female as shown here or bisexual.  This photo shows a prothallus growing on a peat block.

d. Male gametes (sperm) are formed in an  antheridium. The antheridia often form on the underside of the prothallus among the rhizoids [photo]. The female gamete (egg) is formed in an archegonium.  In section one can see the archegonium is composed of neck cells and an egg at the base of the chamber. Water is required to allow the motile sperm to swim to the opening of the archegonium.  Sperm are attracted to the archegonial opening by a chemical attractant.

e. At the moment of fertilization, the nuclei of sperm and egg fuse and a diploid zygote is formed. This begins the sporophytic generation again. The zygote divides mitotically to form an embryo. This embryo develops tiny leaves and is at first still attached to the notch area of the prothallus [young sporophyte] [diagram]. It eventually grows much larger and looses its dependence upon the gametophyte.

See Fern Life Cycle at Land Plants Online Web Site.

1. Figure 8.3 in your text shows our current understanding of relationships among the various monilophyte clades (from Smith et al. 2006).  Note that there are both homosporous and heterosporous groups within monilophyes. The type of sporangium development has traditionally been important in fern classification. Two basic types, those that form a  eusporangium (2-4 below) and those that form a leptosporangium (5-8 below). The eusporangium has two or more cells in cross section (character is plesiomorphic) whereas the leptosporangium has just one cell in cross section.

2. Psilotales. Psilotaceae (2/12).  Whisk Fern Family. Life cycle of Psilotum. Previously classified in its own Division (Psilophyta).  Morphological and molecular evidence indicate a close relationship to the eusporangiate ferns.  Psilotum occurs terrestrially or epiphytically in tropical areas of the world. The other genus is Tmesipteris which is a tropical epiphyte. It looks quite different than Psilotum with its leaflike appendages and axillary sporangia.

• The plant has no roots but the stem is an underground branched rhizome (cross section showing actinostele). 
• The plant has dichotomously branching stems that lack leaves (it does has scale-like enations).
• Psilotum has massive (eusporangiate) sporangia located at axillary positions on the branches. Since three sporangia appear fused to each other, it is sometimes called a synangium. 
• Plants produce one size of spore (homosporous) as shown in this cross section of the synangium.
• The gametophyte also branches dichotomously occurs underground and is associated with mycorrhizal fungi. It forms typical antheridia and archegonia.

See Psilotum Life Cycle at Land Plants Online Web Site.

4.  Marattiaceae.  4-7 genera. Angiopteris, Danaea, and Marattia.  Tropical ferns with large fronds (photo).  Photos of frond, abaxial side of pinna showing eusporangia. Note that along with Equisetum, Angiopteris sister to all leptosporangiate ferns.

3. Ophioglossales. Eusporangiate ferns. Two main genera, Ophioglossum and Botrychium (compared).

a. Ophioglossum. Adder's tongue ferns. Leaves simple, often with dimorphic pinnae (sterile and fertile). Fertile pinnae spikelike.   Ophioglossum has the record for the largest number of chromosomes of any organism (2N > 1200).

b.  Botrychium. Grape ferns and moonworts. Leaves usually as a compound sterile pinna and a paniculate fertile pinna. Common species in S. Illinois are B. virginianum and B. dissectum var. obliquum.

4.  Equisetales.  Horsetails or Scouring Rushes. Life cycle of Equisetum. Previously classified in its own Division (Equisetophyta), sperm ultrastructure and molecular data place Equisetum with the the leptosporangiate ferns. Equisetum is the last surviving genus of a once widespread and diverse division that included the huge, tree-like plants of the Carboniferous.

• Equisetum is herbaceous with a hollow, jointed, longitudinally ridged stem. The epidermal cells contain silicon dioxide and are rough to the touch (hence the name scouring rushes).
• The branches are either absent or present as whorls at the nodes.
• The leaves are scale-like and present at the nodes as a whorl or sheath.
• Sporangia are formed from polygonal peltate scales called sporangiophores [diagram]. These scales, hexagonal as viewed from the surface, are arranged on a terminal axis called a strobilus. Strobilus, fully mature, side view.  Strobilus longitudinal section. Close-up of peltate scales with sporangia.
• Plants are homosporous. The spores of Equisetum are green and surrounded by elaters that aid in dispersal [diagram].

See Equisetum Life Cycle at Land Plants Online Web Site.

5.  Osmundales.  Royal Fern Family. Leptosporangiate and sister to all the other leptosporangiate ferns.  Main genus Osmunda, includes:

• royal fern (Osmunda regalis)
• cinnamon fern (O. cinnamomea). Fertile fronds.
• interrupted fern (O. claytoniana).

Note the variations in the fertile vs. sterile portions (either whole fronds or pinnae on the same frond).

6.  Salviniales.

Salviniaceae. Mosquito ferns. Salvinia and Azolla. Tiny, floating aquatic ferns with dimorphic leaves; float by means of abundant hairs. Plants heterosporous. Azolla has cyanobacteria associated with it - gives it a dark red color. Purposefully "planted" in rice paddies to increase nitrogen.
 
Marsileaceae. Marsilea quadrifolia (Water-clover) and Pilularia. Aquatic ferns with quadrifoliate leaves. Plants heterosporous. Forms sporangia inside a sporocarp (sectioned sporocarp).  Sporangia are strung out along a long, gelatinous extension when the spores are released.

7.  Cyatheales.  Tree ferns.  Large ferns with multiply compound leaves. Cyathea fiddleheads, Alsophila abaxial side of pinnae showing sori completely surrounded by the globose indusium.
 
8.  Polypodiales. Many families. Most of the more common ferns one sees.

a. Dennstaedtiaceae

• Pteridium (Bracken fern), e.g. P. aquilinum - habit.
• Also Dennstaedtia punctilobula (hay-scented fern)
  

b. Pteridaceae

• Adiantum (Maidenhair fern), e.g. A. pedatum habit, A. hispidulum sori.
• Also Pteris, Cheilanthes
  

c.  Aspleniaceae

• Asplenium (Spleenwort), e.g. A. nidus (birdnest fern),  A. rhizophyllum (walking fern), A. platyneuron (ebony spleenwort).

d. Thelypteridaceae

• Thelypteris (Beech fern), e.g. T. novaboracensis

e.  Woodsiaceae

• Woodsia, e.g. W. obtusa
• Athyrium (Lady fern), e.g. A. felix-femina (lady fern); also A. asplenoides.

f.  Onocleaceae

• Onoclea, e.g. O. sensibilis (Sensitive fern)

g.  Dryopteridaceae

• Dryopteris (Shield fern), e.g. D. carthusiana, D. marginalis
• Pellaea (Cliffbrake fern), e.g. P. viridis - frond and close-up showing the false indusium,  Pellaea sp. (S. Africa).
• Phegopteris (Beech fern), e.g. P. hexagonoptera.
• Polystichum (Christmas fern), e.g. P. acrostichoides - habit and close-up of sori.
• Cystopteris (Fragile fern) e.g. C. fragilis and C. bulbifera (Bulblet fern)

h.  Polypodiaceae

• Polypodium  (Resurrection fern), e.g. P. crassifolium showing naked (exindusiate) sori,  P. virginianum a native S. IL species.

SIUC / College of Science / Plant Biology/ PLB 304/ Elements of Systematics

URL: http://www.plantbiology.siu.edu/PLB304/Ferns.html

Last updated: 06-Feb-08 / dln